WASHINGTON (Reuters) -- An experimental AIDS vaccine made from two older versions has made major progress in finding an effective way to combat the illness, researchers said.

But its application may be limited and a commercial vaccine may require more time.

The health minister of Thailand, where the trials were conducted, called the outcome a "very important step for developing an AIDS vaccine." Two UN agencies said they gave "new hope" in battling the disease but more work was needed.

At least one researcher suggested the vaccine might not be effective in areas and in instances where AIDS is most prevalent.

The vaccine is a combination of Sanofi-Pasteur's ALVAC canary pox vaccine and the failed HIV vaccine AIDSVAX, made by a San Francisco company called VaxGen and now owned by the nonprofit Global Solutions for Infectious Diseases.

The trial was sponsored by the U.S. Army and conducted by the Thai Ministry of Public Health.

Officials from the two countries told a news conference in Bangkok the risk of infection had been cut by 31.2 percent among 16,402 volunteers.

"The result of the study is a very important step for developing an AIDS vaccine," Thai health minister Withaya Kaewparadai said. "It's the first time in the world that we have found a vaccine that can prevent HIV infection."

The result puzzled researchers, who said they could not understand why the vaccine combination was working.

Trial Supporters Jubilant

But it was a triumph for supporters, who went ahead with the giant trial despite criticism it was unethical or a waste of money because the vaccine was widely expected to have no effect.

"Myself, like others, did not think there was a very high chance that this would give any degree of efficacy," said Dr. Anthony Fauci of the U.S. National Institute of Allergy and Infectious Diseases, which helped pay for the study.

"But nonetheless, we went ahead with the trial and it was controversial to go ahead with it."

In Geneva, the World Health Organization and the Joint United Nationals Program on HIV/AIDS said: "The study results, representing a significant scientific advance, are the first demonstration that a vaccine can prevent HIV infection in a general adult population and are of great importance."

The two agencies, in a joint statement, described the efficacy as "modestly productive."

"We see no commercial vaccine available for some time yet, but the prospect has finally been raised [after 30 years of trying] that an effective vaccine is possible," said Michael Leacock, an analyst at ABN AMRO research.

More extensive work was needed, he said, before a vaccine could be suitable for regulatory approval.

Results Remain Inconclusive

Muddying the waters of the trial result, people who got the vaccine and who became infected anyway had just as much virus in their blood and just as much damage to their immune systems as HIV patients who went unvaccinated.

That meant the vaccine helped prevent infection but did nothing to affect the virus once it is in the body.

"We had 74 infections in the placebo group and 51 in the vaccine group," Dr. Jerome Kim, a U.S. Army colonel at the Walter Reed Army Institute of Research in Maryland, who helped lead the trial, said by telephone.

"Although the level of protection that we saw was clearly modest, the study is a major scientific advance," Kim said.

"It is the first evidence that the development of a safe and effective vaccine is possible. Although we don't have all the answers now, it does have important implications for the future of HIV vaccine design."

Kim said the vaccine might not work in the people and places where HIV is most common -- in Africa, among men who have sex with men and among injecting drug users.

"The vaccine was tested in Thailand and it is really specific for the strains that are circulating in Thailand now," Kim said.

Both Fauci and Kim noted that the vaccine was formulated specifically to work against two subtypes of the human immunodeficiency virus -- E, which circulates in Thailand and Southeast Asia, and B, common in the United States and Europe.

In a statement, Sanofi-Aventis Chief Executive Chris Viehbacher said that the company would continue its research into HIV in partnership with with academics, governments, nongovernmental organizations and other vaccine makers.

The volunteers in the trial got six immunizations over six months, four with ALVAC, and two with AIDSVAX.

ALVAC is a genetically engineered canarypox virus that has spliced into it synthetic versions of three HIV genes. AIDSVAX is made using two versions of one HIV gene, one from the B subtype and one from the E subtype.

The AIDS virus infects an estimated 33 million people globally and has killed 25 million since it was identified in the 1980s. It affects immune cells called T-cells.

Cocktails of drugs can control the virus but there is no cure. In 2007, Merck & Co ended a trial of its vaccine after it was found not to work, and in 2003, AIDSVAX used alone was found to offer no protection, either.